Recently, as depletion of conventional energy resources such as oil or coal is foreseen, interest in an alternative energy is increasing. A fuel cell is one of the alternative energy, and advantageously has a high efficiency, does not emit pollutants of NOX and SOX and uses a fuel that is abundant in quantity, and therefore, the fuel cell attracts public attention.
The fuel cell is a power generation system that converts chemical energy of a fuel and an oxidant to electrical energy, and typically hydrogen and hydrocarbon, for example methanol or butane is used as a fuel, and oxygen is used as an oxidant.
In the fuel cell, a membrane electrode assembly (MEA) is the basic unit for generating electricity, and includes an electrolyte membrane and anode and cathode electrodes formed at opposite sides of the electrolyte membrane. FIG. 1 illustrates the principle for generating electricity of a fuel cell, and Chemical FIG. 1 represents a reaction formula of a fuel cell in the case that hydrogen is used as a fuel. Referring to FIG. 1 and Chemical FIG. 1, an oxidation reaction of a fuel occurs at an anode electrode to generate hydrogen ions and electrons, and the hydrogen ions move to a cathode electrode through an electrolyte membrane. The hydrogen ions transmitted through the electrolyte membrane and the electrons react with oxygen (oxidant) at the cathode electrode to generate water. This reaction causes the electrons to move to an external circuit.Anode electrode: H2→2H++2e−cathode electrode: ½O2+2H++2e−→H2OReaction formula: H2+½O2→H2O  [Chemistry Figure 1]
FIG. 2 illustrates a general configuration of a membrane electrode assembly for a fuel cell. Referring to FIG. 2, a membrane electrode assembly for a fuel cell includes an electrolyte membrane, and an anode electrode and a cathode electrode located at the opposite sides of the electrolyte membrane. The anode and cathode electrodes respectively include a catalyst layer and a gas diffusion layer. The gas diffusion layer includes an electrode substrate and a microporous layer formed on the electrode substrate.
In the fuel cell, the moisture that is also a resultant material of reaction in an electrode assists ion transfer, but an excessive amount of moisture may block the micro pores in the catalyst layer or the gas diffusion layer. Namely, the moisture discharging ability in the electrode surface of a fuel cell is a factor determining performance of the cell. Thus, the moisture introduced into the electrode or the moisture generated in the electrode should be controlled suitably. If moisture is not discharged suitably, flooding occurs, which decreases the three-phase reaction sites and reduces an activation area of catalyst, thereby deteriorating the efficiency of the fuel cell.
However, a catalyst layer of a conventional fuel cell membrane electrode assembly is made by coating with one kind of ink including a catalyst and an ionomer, so the same catalyst layer is formed entirely. Thus, it was difficult to control water repelling in the catalyst layer.
In this regards, Japanese Laid-open Patent Publication No. 2006-286330 discloses a method for making an electrode with different hydrophile properties in a catalyst layer by surface-reforming catalyst particles with compounds with different hydrophile properties, but this method does not still solve the above problems in an effective way.